Copending application Ser. No. 129,435, now U.S. Pat. No. 4,313,376, filed on Mar. 11, 1980 and owned by the Assignee of this application, discloses an imprinter wherein a film or ribbon is intermittently withdrawn from a rotatable spool in accordance with the demand or usage of the ribbon. The imprinter has an adjustable brake device associated with the spool for imposing a braking torque thereon so as to prevent or control inertia-induced overrunning of the spool after the intermittent pulling-off of the ribbon has been terminated. This known braking mechanism employs a cam which coacts between a follower and a brake actuator so that the brake torque is a maximum at large spool diameters and progressively decreases as the spool becomes empty. While this brake mechanism has performed in a manner which is generally satisfactory, nevertheless it has been observed that the rate of brake torque decrease and the difference between the maximum and minimum brake torques always remains the same for the same spool diameter. While the initial or minimum brake torque (corresponding to the empty spool condition) can be initially adjusted in an attempt to compensate for ribbon spools of different inertia (such as spools of different axial length), nevertheless this adjustment affects only the initial or minimum torque since the rate or torque change and the range of torque (that is, the difference between the maximum and minimum torques) are not affected. In such circumstance, when the minimum or initial torque is increased so as to compensate for a heavy spool (that is, a spool of substantial axial length), then while this initial adjustment results in the maximum and minimum torques being both shifted upwardly, nevertheless the range or differential magnitude between these maximum and minimum torques still remains the same, as does the rate of torque change. Thus, under such circumstances, the brake torque imposed on the spool is not optimized since the adjustment must be such so as to thus result in substantially greater torque than desired when the spool approaches an empty condition (rather than the torque approaching zero as is desired for optimum operation) so that the proper performance is hence often seriously impaired. Since spools of the same diameter may have widely varying axial lengths, which different axial lengths may be of several orders of magnitude, and since the density of the film or ribbon itself may significantly vary without affecting the maximum spool diameter, these factors hence have a significant effect on the inertia and hence overrunning tendency of the spool. For this reason, the known brake mechanisms, one form of which is illustrated in the aforesaid copending application, have hence been unsuitable for use under such variable conditions since these known mechanisms have basically adjusted solely for changes in diameter.
Accordingly, this invention relates to an improved adjustable brake mechanism adapted for cooperation with a spool for imposing a braking torque thereon so as to prevent overrunning of the spool as the film or ribbon is pulled intermittently therefrom. The improved brake mechanism of this invention is hence intended to overcome the disadvantages associated with the known mechanisms, as explained above.
In the improved brake mechanism of this invention, the mechanism again includes structure which progressively decreases the braking torque on the spool as the outer diameter of the spool decreases due to withdrawal of ribbon or film. In addition, this improved brake mechanism incorporates adjustment structure therewith which changes both the rate of torque change and the range (that is, the differential between the maximum and minimum torques) without affecting the zero or minimum torque which exists under the empty spool condition. In this manner, the adjustment of the braking mechanism hence enables the braking torque to be suitably adjusted for use with numerous spools which, while of the same diameter, may be of substantially different inertia due to their being of different axial lengths or of different ribbon densities. In this manner, the braking torque imposed on the spool can hence be optimized so as to progressively decrease approximately linearly from the full-spool condition to the empty-spool condition, with the torque always approaching a very small minimum brake torque (which is preferably equal to or only slightly greater than zero) so as to thereby provide more uniform control over the pulling off of ribbon from the spool throughout the complete range between the full and empty conditions of the spool.
The improved brake mechanism includes a brake member disposed in braking relationship with the spool or its support shaft for retarding rotation thereof, which brake member has a brake actuator projecting therefrom. An elongated follower or sensor is pivotally supported on the frame and bears against the outer diameter of the spool so as to follow the decrease in the diameter thereof. This follower has an adjustment means thereon which creates a camming relationship with the brake actuator for progressively moving same and decreasing the brake torque as the follower pivotally moves in response to a decrease in the spool diameter. This adjustment means is positioned so as to effectively contact the brake actuator and impose either a zero or a minimum base torque on the brake member when the spool is empty. When in this empty-spool condition, the adjustment means can be initially adjusted in a direction substantially parallel to the radial direction of the brake actuator so as to not affect this zero or minimum brake torque. However, this change in the position of the adjustment means causes the latter to swing about a different arc when the follower pivots due to being engaged with the full ribbon spool, whereby the movement imposed on the brake actuator is correspondingly changed so that the maximum brake torque for any selected spool diameter can hence be adjusted without affecting the zero or minimum torque for the empty-spool condition. The rate of torque decrease is hence adjusted so as to provide optimum control as a function of the spool inertia.
Other objects and purposes of the invention will be apparent to persons familiar with devices of this type upon reading the following specification and inspecting the accompanying drawings.